In Appearance > Menus you can create a login menu using custom links and can enter the corresponding login page link in the url field .
You can try out this plugin – https://wordpress.org/plugins/theme-my-login/ to get more features .
Or
You can just add the below code in your childtheme’s functions.php and see if it helps .
add_filter('wp_nav_menu_items', 'add_login_logout_link', 10, 2); function add_login_logout_link($items, $args) { ob_start(); wp_loginout('index.php'); $loginoutlink = ob_get_contents(); ob_end_clean(); $items .= '<li>'. $loginoutlink .'</li>'; return $items; }